Electromechanical motion multiplying devices, using stiff, compact, short-travel, low inertia actuators are used in a wide range of systems where high speed or high resolution mechanical movement is essential such as in laser beam targeting, telescope light beam tracking, vibration control, and scanning applications. These devices typically employ piezoelectric, electrostrictive, or electromagnetic actuators which act either directly or via leveraging means.
Certain optical systems require high speed low inertia, low hysteresis tilting devices for the precision 2-axis control of mirrors, optics or primary light beam sources. Some applications require both a large range of motion and high stiffness with minimal undesirable resonances. It will be seen that such existing or previously patented devices generally sacrifice stiffness for increased range of motion. Generally the inventor is forced to make some trade-off which might be undesirable for a range of applications. Ideally a device could be adaptable to satisfy a wide variety of applications and would be readily customized to increase tilting range while maintaining needed stiffness.
High speed scanning, pointing or steering devices should be capable of billions of cycles without developing any mechanical wear or “slop,” would never require lubrication and would preferably employ flexural elements to drive and control the motion of the mechanism, in lieu of mechanical bearings. Such devices would preferably actuate without undesired cross coupling of motions and would be stiff in all axes of motion except those desired and commanded by the control system. Such devices may require high resonant stiffness when motionless to maintain a desired position in the presence of external vibrations in the environment, such as might be induced by gusting winds or adjacent vibrating machinery.
Manufacturing cost is an issue in certain high volume products. Laser communication devices manufactured in production lots will require a precise 2-axis beam steering mirror or laser source pointing device which could be readily and cheaply manufactured and would preferably contain few parts.
To maximize tilting range a device may require very thin flexural elements which are complex and difficult to manufacture by common methods due to bending of relatively long and thin elements from tool cutter forces. Electrical discharge machining (EDM) methods eliminate cutting tool forces, however large quantities cannot be economically machined using this method owing to the slow rate of material removal. Very thin flexural elements would ideally be produced by high speed stamping from pre-hardened, ultra-high strength sheet stock, thus eliminating machining and post-hardening operations of thin sections.
The present invention makes use of conventionally milled and machined integral flexures where relative movement is small. High strength flexures stamped from sheet stock would ideally be used to accommodate large tilting or bending motions in the device to maximize flexure life.
Existing low-cost scanning and steering devices achieve some of the desired characteristics described above. Economically priced, commercially available devices generally consist of three piezoelectric actuators operating directly on a mounting platform which is restrained radially by a surrounding housing and axially by a rod flexure or screw. These conventional devices exhibit fair stiffness but with limited range.
The designers of these lower cost devices tend to employ hysteresis-prone sliding fits in lieu of friction-free flexural guidance in order to reduce cost. In order to eliminate bending in fragile piezoelectric (PZT) actuators made of ceramic, designers of such lower cost devices employ a hardened ball to eliminate bending in the actuator which comprises a mechanical joint subject to wear, fretting, and hysteresis.
Various recently marketed “fast steering mirrors” driven by voice coil type linear actuators offer angular tilts exceeding 0.5 degrees. Newport Corporation (Irvine, Calif.) offers a custom made device, and advertises high range and flexural pivots. Voice coil actuators, however, are less compact than are electrostrictive or piezoelectric actuators and have no inherent stiffness. Therefore they must be stiffened through electrical damping means with a corresponding increase in complexity of controls and loss of electrical efficiency.
Liddiard in U.S. Pat. No. 4,708,420, discloses a device having high range and speed but not stiffness as an objective. Patel, in U.S. Pat. No. 5,550,669, discloses a complex device claiming stiffness and range of motion as apparent primary objectives, but not simplicity or ease of manufacture.
Laurer, in U.S. Pat. No. 4,436,634, invented a simple high displacement device in 1984 which consists of two oppositely oriented bimorph piezoceramic strips attached to a mirror. The Laurer device could benefit greatly from the addition of a rod flexure employed in the present device. Ellis, in U.S. Pat. No. 4,203,654 has a number of outboard rod flexures controlling piston motion, but these are connected to leaf type actuators so that the device must employ a centroidal pivot apparently to maintain resonant stiffness. U.S. Pat. No. 4,973,145 employs relatively long rod flexure supports arranged in a conical fashion, enabling considerable tilt and low hysteresis, but permitting low frequency tilting resonances due to lack of restraints to control the six possible axes of motion in a mechanical structure. Tilting resonances could be eliminated by electronic damping of the peripheral actuators, but the device seems unsuited to high speed operation.
From the foregoing discussion therefore, it is clear that prior art inventions and apparatus do not ideally meet the competing requirements of high stiffness, high range, low hysteresis, producibility, and specification adaptability. As noted hereinbelow, the range of the present inventive device may be increased by simply installing a thinner central spacer, or the resonant stiffness may be alternatively increased by installing a thicker spacer, where some loss of range is acceptable. Unlike the devices cited, the present device employing inherently stiff actuators relies on tensile, shear or compressive stiffness, as opposed to bending stiffness, to maximize operational bandwidth.
It is therefore an object of the invention to provide the most efficient balance of stiffness and range of motion for a number of steering, scanning and pointing applications.
It is another object of the invention to derive maximum benefit from compact, inherently stiff, precisely controllable, low inertia, high force actuators such as piezoelectric, magnetostrictive and electrostrictive actuators.
It is another object of the invention to facilitate high volume production of the inventive device by employing a relatively small number of easily manufactured elements.
It is another object of the invention to provide extended life and range of the device by employing certain flexural elements stamped from high performance alloy sheet stock.
It is another object of the invention to facilitate the use of the diecastable or freely machined materials in the manufacture of the base structure and certain integral, moderately stressed, linear motion flexures.
It is another object of the invention to provide a ready means of meeting different marketplace applications while maximizing commonality of elements.